For decades, the source of Earth's volatiles, especially water with a deuterium-to-hydrogen ratio (D/H) of (1.558 ± 0.001) × 10−4, has been a subject of debate. The similarity of Earth’s bulk composition to that of meteorites known as enstatite chondrites1 suggests a dry proto-Earth2 with subsequent delivery of volatiles3 by local accretion4 or impacts of asteroids or comets5,6. Previous measurements in six comets from the Oort cloud yielded a mean D/H ratio of (2.96 ± 0.25) × 10−4. The D/H value in carbonaceous chondrites, (1.4 ± 0.1) × 10−4, together with dynamical simulations, led to models in which asteroids were the main source of Earth's water7, with ≤10 per cent being delivered by comets. Here we report that the D/H ratio in the Jupiter-family comet 103P/Hartley 2, which originated in the Kuiper belt, is (1.61 ± 0.24) × 10−4. This result substantially expands the reservoir of Earth ocean-like water to include some comets, and is consistent with the emerging picture of a complex dynamical evolution of the early Solar System8,9.
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Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation by NASA. The Heterodyne Instrument for the Far Infrared (HIFI) has been designed and built by a consortium of institutes and university departments from across Europe, Canada and the United States under the leadership of SRON, the Netherlands Institute for Space Research, and with major contributions from Germany, France and the USA. This development has been supported by national funding agencies: CEA, CNES, CNRS (France); ASI (Italy); and DLR (Germany). Additional funding support for some instrument activities has been provided by ESA. Support for this work was also provided by NASA through an award issued by JPL/Caltech. D.C.L. is supported by an NSF award to the Caltech Submillimeter Observatory. We thank R. Lorente, P. García-Lario, M. Kidger and G. Pilbratt for helping with the scheduling of these observations, and I. Avruch for the assistance with HIFI specific software issues.
The authors declare no competing financial interests.
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Hartogh, P., Lis, D., Bockelée-Morvan, D. et al. Ocean-like water in the Jupiter-family comet 103P/Hartley 2. Nature 478, 218–220 (2011). https://doi.org/10.1038/nature10519
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